One method for increasing bandwidth of an operational amplifier (opamp) is to use a low-gain, high-bandwidth amplifier as an input amplifier preceding a regular opamp, which forms a composite opamp. For example, one suitable composite opamp is described in “A Pipelined 13-bit, 250-ks/s, 5-V Analog-to-Digital Converter,” Sehat Sutardja and Paul Gray, IEEE Journal of Solid-State Circuits (1988), which is hereby incorporated by reference in its entirety. The low-gain input amplifier of the composite opamp may be implemented using a gm*R or gm/gm configuration. That is, the input amplifier uses a resistor or a transistor as a load. This input amplifier, however, has a limited gain bandwidth product.
Referring now to FIGS. 1 and 2, a composite opamp 10 includes a transistor 14 having a gate coupled to an input voltage Vin. One terminal of the transistor 14 is coupled to a current source 18. A second terminal of the transistor 14 is coupled to a resistive load or resistor 16 and to a gate of a transistor 20. The resistor 16 and a first terminal of the transistor 20 are biased by a voltage source Vdd. A second terminal of the transistor 20, which acts as a follower, is coupled to an opamp 30. In FIG. 2, the resistor 16 may be replaced by an active load or a transistor 38 such as a diode-connected transistor.